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EL expressions are designed to make a ZUML document easier to access objects available in the application, such as the application data and parameters.

An EL expressions is an expression enclosed with ${ and }, i.e., the syntax ${expr}. For example,

 <element attr1="${}".../>
 <another-element>${3+counter} is ${empty map}</another-element>

When an EL expression is used as an attribute value, it could return any kind of objects as long as the attribute allows. For example, the following expressions will be evaluated to boolean and int respectively.

 <window if="${some > 10}"><!-- boolean -->
   <progressmetter value="${progress}"/><!-- integer -->

If the class does not match, ZK Loader will try to coerce it to the correct one. If a failure has occurred, an exception is thrown.

Multiple EL expressions could be specified in a single attribute:

<window title="${}: ${foo.version}">


EL Expression Result
${1 > (4/2)} false
${100.0 == 100} true
${'a' < 'b'} true
${'hip' gt 'hit'} false
${1.2E4 + 1.4} 12001.4
${3 div 4} 0.75
${10 mod 4} 2
${empty param.add} true if the request parameter named add is null or an empty string
${param['mycom.productId']} The value of the request parameter named mycom.productId

Difference from Java

  • A string can be enclosed with either single quotes or double quotes. In other words, 'abc' is equivalent "abc".
  • The empty operator is useful for testing null and empty string, list and map, such as ${empty param.add}.
  • The . operator can be used to access a property of an object (assuming that there is a get method of the same name) or a value of a map, such as ${}.
  • The [ ] operator can be used to access an item of a list or array, a value of a map, and a property of an object (assuming that there is a get method of the same name), such as ${ary[5]} and ${wnd['title']}.
  • null is returned if the value is not found and the index is out-of-bound.

For more information please refer to Operators and Literals.

Connecting to Java World

EL expressions are evaluated on the server when the page is rendered. Thus, it is allowed to access:

<window title="EL">
    <textbox id="tb" value="${self.parent.title}"/> <!-- self is an implicit object referring to itself -->
    ${tb.value} <!-- tb is an ID (of textbox) -->
    <button label="Enter" if="${not empty param.edit}"/>
    <zscript>Date now = new Date();</zscript>
    <datebox value="${now}"/> <!-- now is defined in zscript -->

Furthermore, you could define a variable resolver to associate a name with an object, or map a function to a Java static method as described in the following.

Variable Resolver

If you would like to support many variables, you could implement a variable resolver: a class that implements VariableResolver.

package foo;
public class CustomerResolver implements org.zkoss.xel.VariableResolver {
    public Object resolveVariable(String name) {
        if ("customers".equals(name))
            return Customer.getAll("*");
//     if ("recent".equals(name))
//         return something_else;
        return null; //not a recognized variable

Then, you could specify it in a variable-resolver directive, such as:

<?variable-resolver class="foo.CustomerResolve"?>

    <listitem label="${}" forEach="${customers}"/>

System-level Variable Resolver

If you have a variable resolver that will be used on every page, you can register a system-level variable resolver rather than specifying it on every page.

This can be done by specifying a variable resolver you have implemented in WEB-INF/zk.xml as follows. For more information, please refer to ZK Configuration Reference.


Then, when a page is created each time, an instance of the specified class will be instantiated and registered as if it is specified in the variable-resolver element.

Notice that since a new instance of the variable resolver is created on each page, there will not be any concurrency issues.

Associate with a Java Method

The collection object could be retrieved by invoking a static method. For example, suppose that we have a class and a static method as follows:

package foo;
public class Customer {
    public static Collection<Customer> getAll(String condition) {
        //...returns a collection of customers
    public String getName() {
       return _name;

Then, we could retrieve them with the xel-method directive:

<?xel-method prefix="c" name="getAllCustomers" class="foo.Customer"
   signature="java.util.Collection getAll(java.lang.String)"?><!-- Generics not allowed -->
    <listitem label="${}" forEach="${c:getAllCustomers('*')}"/>

Associate with Multiple Java Methods

If you have several static methods, you could declare them in a XML file called taglib, such as

	java.util.Collection getAll(java.lang.String)
	Returns a collection of customers.
	<!-- any number of functions are allowed -->

Then, you could use them by specifying it in a taglib directive.

<?taglib uri="/WEB-INF/tld/my.tld" prefix="my"?>
    <listitem label="${}" forEach="${my:getAllCustomers('*')}"/>

Version History

Last Update : 2011/9/19

Version Date Content

Copyright © Potix Corporation. This article is licensed under GNU Free Documentation License.

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